Doubts have been cast on the possibility of a lake of liquid water buried beneath Mars’ southern ice cap by new computer simulations that suggest that tightly packed layers of ice can produce the same radar reflections that liquid water would.
In 2018, the European Space Agency Mars Express orbiter used its MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument to identify what appears to be 20 kilometers (12.4 miles) wide a lake of liquid water buried deep under 1.5 km (0.93 mi) of ice in a region called Planum Australe in the south polar plain of Mars. Subsequently, similar evidence emerged for potentially dozens of lakesbut some are so close to the surface that it seems impossible for water to be a liquid there.
This is because the surface of Mars is Too cold and on atmospheric too low a pressure to allow liquid water to stay too close to the surface. However, at the base of the southern polar ice cap, the temperature and pressure conditions, with the help of a little natural antifreeze, could allow salt lakes to exist.
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This antifreeze can be in the form of calcium and magnesium perchlorate, which is a chemical compound found on the surface of Mars by NASA phoenix mission in 2008, magnesium and calcium perchlorate, when dissolved in water, would lower their freezing points to a minimum of minus 68 degrees Celsius and minus 75 degrees Celsius (minus 92 and minus 103 degrees Fahrenheit), respectively—very close to the predicted temperature of minus 68 degrees C (minus 90 degrees F) at the base of the ice cap. Therefore, it is not too difficult to imagine localized conditions of temperature, pressure, and perchlorate concentration conspiring to allow large pools of liquid water on Mars.
Further evidence for such lakes comes from measurements of surface ice ripples; the liquid water reduces the amount of friction between the ice sheet and the bedrock beneath it, allowing the ice sheet to flow faster over the bedrock. This increase in flow rate leads to dips and peaks in the surface ice, which is exactly what is seen in the Planum Australe.
Despite all this evidence, however, many in the planetary science community are skeptical; the presence of liquid water on Mars would be an extraordinary find and requires extraordinary evidence. Now, a team of scientists at Cornell University is fanning the flames of that skepticism with new findings that provide an alternative explanation for radar echoes.
“I can’t say it’s impossible to have liquid water down there, but we’re showing that there are much simpler ways to get the same observations without having to stretch so far using mechanisms and materials, that we already know exist there,” said Cornell’s Daniel Lalich in a statement. Lalich is the lead author of new research that suggests compacted ice sheets can return a strong radar signal that looks just like the radar echo from a layer of liquid.
A large body of water is able to reflect the radar back to its source because of how flat the lake is, etc The Earth bright radar reflections of the kind detected by MARSIS would almost certainly indicate liquid water similar to pockets of water beneath Antarctica, such as Lake Vostok. However, planetary scientists must be careful to assume that what is true for Earth is also true for other planets where conditions are not the same.
Lalich’s group ran thousands of simulations to test whether multiple densely packed layers of ice could mimic the radar signal of a lake. Each simulation changes both the thickness of the ice sheets and their composition (meaning how dirty they are). They found that in many cases, tightly packed layers of ice, deposited long ago and crushed under the weight of the ice sheet, can produce bright radar reflections just like those detected by MARSIS.
The trick is “constructive interference” of radar waves. The spatial resolution of MARSIS is limited, and if the ice layers are too thin, the radar instrument cannot distinguish them. Each layer would reflect back a portion of the radar beam, and because the layers are squished so tightly together, the radar echoes overlap and combine, amplifying their strength and making them appear brighter.
“This is the first time we have a hypothesis that explains the entire population of observations under the ice cap without having to introduce anything unique or strange,” Lalich said. “This result, where we get bright reflections scattered all over the place, is exactly what you would expect from thin-layer interference in radar.”
For now, the question of whether there is a salt lake under the south polar cap remains unanswered, but Lalich argues that the simulations at least provide a much simpler and, he says, more likely explanation than a lake.
“The idea that there would be liquid water even slightly close to the surface would be really exciting,” Lalich said. “I just don’t think it’s there.”
Lalich’s team’s findings were published June 7 in the journal Scientific progress.
